Preparation of 2,3-dimethylnaphthalene

ABSTRACT

A MULTISTEP PROCESS IS DISCLOSED WHEREIN 5-PHENYLHEXENE-2 IS CONVERTED TO 1,4-DIMETHYLETRALIN IN THE PRESENCE OF A SOLID ACIDIC CATALYST AT 200-450*C. AND PREFERABLY ALSO RECYCLED HYDROGEN, VAPORS FROM THIS CONVERSION ARE REACTED BY CONTRAST AT 300-500*C. IN A HYDROGEN ATMOSPHERE WITH A SOLID DEHYDROGENATION CARALYST, PREFERABLY PLATINUM ON NON-ACIDIC ALUMINA, TO FORM 1,4-DIMETHYLNAPHTHALENE AND HYDROGEN, VAPORS FROM THE DEHYDROGENATION REACTION ARE CONTACTED AT 275-500*C. WITH A SOLID ACIDIC ISOMERIZAION CARALYST TO FORM 2,3-DIMETHYLNAPHTHALENE IN ADMIXTURE WITH 1,3-DIMETHYLNAPHTHALENE, 2,3-DIMETHYLNAPHTHALENE IS RECOVERED FROM THE ISOMERIZATION PRODUCT, E.G. BY CRYSTALLIZAION, AND PART OF THE GENERATED HYDROGEN IS RECYCLED IN THE SYSTEM. THE PROCESS CAN BE VARIED IN SEVERAL WAYS, INCLUDING THE UTILIZATION OF A LIQUID ACIDIC CATALYST IN EITHER THE FIRST STEP OR THE ISOMERIZATION STEP.

Nov. 27, 1973 v 5. L. THOMPSON 3,775,497

PREPARATION OF I I3DIMETHYLNAPH'IHALENE Filed Aug. 9, 1972 2 Sheets-Sheet I A: CYCLIZATION g$ERAT|ON 23 B DEHYDROGENATION --2-;

C ISOMERIZATION FIG. 2

SEPARATION I 53 LI 2,3-DMN Nov. 27, 1973 s. 1.. THOMPSON PREPARATION OF 2 Z-DIMETHYLNAPHTHALENE 2 Sheets-Sheet 2 Filed Aug. 9, 1972 H RECYCLE D E V E F United States Patent O 3,775,497 PREPARATION OF 2,3-DIMETHYLNAPHTHALENE Sheldon L. Thompson, Glen Mills, Pa., assignor to Sun Research and Development (30., Marcus Hook, Pa. Filed Aug. 9, 1972, Ser. No. 278,899 Int. Cl. C07c 15/24 U.S. Cl. 260668 F 24 Claims ABSTRACT OF THE DISCLOSURE A multistep process is disclosed wherein S-phenylhexene-Z is converted to 1,4-dimethyltetralin in the presence of a solid acidic catalyst at 200-450 C. and preferably also recycled hydrogen, vapors from this conversion are reacted by contact at 300-500 C. in a hydrogen atmosphere with a solid dehydrogenation catalyst, preferably platinum on non-acidic alumina, to form 1,4-di-. methylnaphthalene and hydrogen, vapors from the dehydrogenation reaction are contacted at 275-500 C. with a solid acidic isomerization catalyst to form 2,3-dimethylnaphthalene in admixture with 1,3-dimethylnaphthalene, 2,3-dimethylnaphthalene is recovered from the isomerization product, e.g. by crystallization, and part of the generated hydrogen is recycled in the system. The process can be varied in several ways, including the utilization of a liquid acidic catalyst in either the first step or the isomerization step.

CROSS-REFERENCES TO RELATED APPLICATIONS My copending U.S. application Ser. No. 263,731, filed June 9, 1972, relates to the preparation of 2,6-dimethylnaphthalene from -o-tolylpentene-2 in a combination process wherein the steps are generally analogous to those in the process of the present invention.

My copending U.S. application Ser. No. 278,898, filed Aug. 9, 1972, and entitled, Preparation of 2,6-dimethylnaphthalene and 2,7-dimethylnaphthalene, relates to the conversion of 5-m-tolylpentene-2 to the 2,6- and 2,7- isomers of dimethylnaphthalene in an analogous series of steps.

My copending U.S. application Ser. No. 278,897, filed Aug. 9, 1972, and entitled, Preparation of 2,7-dimethylnaphthalene, relates to the use of an analogous series of steps for converting 5-p-tolylpentene-2 to 2,7-dimethylnaphthalene.

U.S. application Ser. No. 207,870, filed Dec. 14, 1971, by I. A. Hedge describes the use of crystalline zeolite catalysts, which preferably are of the faujasite cage structure and preferably contain rare earth components incorporated by ion exchange, for hydroisomerization of dimethylnaphthalenes (DMNs).

U.S. application Ser. No. 208,001, filed Dec. 14, 1971, by G. Suld and R. L. Urban, likewise discloses the hydroisomerization of DMNs employing a calcium-containing crystalline zeolite catalyst preferably of the faujasite cage structure. A

BACKGROUND OF THE INVENTION This invention relates to the conversion of S-phenylhexene-Z to 2,3-dimethylnaphthalene. The invention particularly concerns a multistep process wherein 5-phenylhexene-2 is first converted to 1,4-dimethyltetralin, the latter is dehydrogenated to 1,4-dimethylnaphthalene (hereinafter referred to as 1,4DMN) and this product is then isomerized to form 2,3-dimethylnaphthalene (2,3-DMN).

The use of 2,3-dimethylnaphthalene (2,3DMN) to make the corresponding naphthalene dicarboxylic acid or anhydride having utility in the preparation of polymers is known, as shown in United States Patent 3,428,698, issued Feb. 18, 1969, H. J. Peterson. This patent also teaches that 2,3-DMN can be partially reduced so as to selectively ice hydrogenate the unsubstituted ring and yield 6,7-dimethyltetralin and further that the latter can be oxidized to pyromellitic acid (1,2,4,5-benzene tetracarboxylic acid) or pyromellitic dianhydride which has known utility for making polyimide resins.

A procedure for making 1,4-DMN employing butadiene and ethylbenzene has been described by G. G. Eberhardt et al. in J. Org. Chem., 30, 82-84 (1965) and in Eberhardt United States Patent 3,244,758, issued Apr. 5, 1966. In this procedure the reactants are interacted in the presence of an alkali metal catalyst under conditions that give mainly the one-to-one adduct, namely, 5-phenylhexene-2. This product is treated with an acidic alkylation catalyst to efiect ring closure and yield 1,4-dimethyltetralin. The latter is then dehydrogenated to produce 1,4-DMN using, for example, platinum-on-alumina as catalyst.

The isomerization reactions of various DMNs employing HF or HF-BF as catalyst have been described by G. Suld et al., J. Org. Chem., 29, 2939-2946 (1964) and in Suld U.S. Pat. 3,109,036, issued Oct. 29, 1963. These references show that isomerization of 1,4-DMN by means of these catalysts will produce 1,3-DMN and 2,3-DMN but substantially no other isomers.

U.S. Pat. 3,336,411, issued Aug. 15, 1967, A. L. Benham, discloses the use of silica-alumina or silica-aluminazirconia catalysts in the presence of hydrogen at temperatures of 250-400 C. for the isomerization of mixed DMNs.

Selective crystallization has been used to separate DMN isomers from each other, although heretofore the 2,6- isomer usually has been the one selectively crystallized as illustrated by the following U.S. patents: M. E. Peterkin et a1. 3,485,885, issued Dec. 23, 1969; I. A. Hedge 3,541,175, issued Nov. 17, 1970; T. E. Skarada et al. 3,590,091, issued June 29, 1971; and J. A. Hedge 3,594,- 436, issued July 20, 1971. However, U.S. Pat. 3,428,698, previously cited, shows the selective crystallization of 2,3-DMN from a mixture thereof with 1,2-DMN.

The separation of DMN isomers from each other by selective complexation with various compounds is disclosed in the following U.S. patents which issued May 23, 172: R. I. Davis et a1. 3,665,043 and 3,665,045 and K. A. Scott 3,665,044.

The prior art also discloses the selective separation of various DMN isomers from each other by selective adsorption on certain types of crystalline zeolites, as shown in U.S. Pat. 3,114,782, issued Dec. 17, 1963, R. N. Fleck et al., and U.S. Pat. 3,668,267, issued June 6, 1972, J. A. Hedge.

U.S. Pat. 3,243,469, issued Mar. 29, 1966, A. Schneider, teaches the use of a platinum-on-alumina catalyst for dehydrogenating dimethyldecalin to produce DMN; and previously cited U.S. Pat. 3,428,698 shows the use of this same kind of catalyst for dchydrogenating a mixture of 2,3- and 1,2-dimethyltetralin.

U.S. Pat. 3,255,268, issued June 7, 1966, G. Suld et al., and U.S. Pat. 3,325,551, issued June 13, 1967, G. Suld, both teach the preparation of non-acidic dehydrogenation catalysts by treating platinum-on-alumina with a solution of a basic salt such as lithium carbonate.

SUMMARY OF THE INVENTION The present invention provides efiicacious procedures for converting 5-phenylhexene-2 to 2,3-DMN by a continuous multistep process involving sequential steps of cyclization, dehydrogenation and isomerization. The process utilizes hydrogen produced in the dehydrogenation step in at least one of the other steps and advantageously in both the dehydrogenation and isomerization steps.

In one aspect the invention provides a process which comprises:

(A) Passing a feed composed mainly of 5-phenylhexene-2 in vapor form over a solid acidic catalyst at a temperature in the range of 200-450" C. and a pressure of -500 p.s.i.g. to effect cyclization and form 1,4-dimethyltetralin;

(B) Contacting the reaction product from Step (A) in vapor form with a solid dehydrogenation catalyst at a temperature in the range of BOO-500 C., in the presence of hydrogen and at a pressure in the range of 0-500 p.s.i.g. to convert 1,4-dimethyltetralin to 1,4-dirnethylnaphthalene by dehydrogenation;

(C) Contacting the resulting vapor mixture of 1,4- DMN and hydrogen with a solid acidic isomerization catalyst at a temperature in the range of 275500 C. and a pressure in the range of O-500 p.s.i.g. to form a mixture of dimethylnaphthalenes including 2,3-dimethylnaphthalene;

(D) Separating hydrogen from the reaction product containing 2,3-dimethylnaphthalene; and

(E) Recycling separated hydrogen to a step in the process preceding Step (C), or in other words to either Step (A) or Step (B).

In another embodiment Step (A) is carried out with the feed in liquid phase employing either a liquid or solid acidic cyclizing catalyst, Steps (B), (C) and (D) are effected as above-described, and separated hydrogen is recycled back to Step (B).

In still another embodiment the dehydrogenation reaction product (1,4-DMN) from Step (B) is condensed and separated from the associated hydrogen gas, the hydrogen is recycled to Step (A), and the 1,4-DMN is isomerized in a subsequent step to form 2,3-DMN.

In any of the embodiments the mixed DMNs from the isomerization step are treated to selectively remove the 2,3-DMN from isomeric DMN (preponderantly 1,3- DMN) and the latter preferably is recycled to the isomerrzer.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is described with reference to the accompanying drawings wherein:

FIGS. 1 and 2. are schematic flowsheets illustrating different ways of practicing the process; and

FIG. 3 is a diagrammatic illustration of a preferred manner of carrying out the process.

DESCRIPTION The 5-phenylhexene-2 feed for the present process can be obtained by the reaction of butadiene and ethylbenzene according to procedures described in the Eberhardt references above cited. This reaction tends to produce minor amounts of higher adducts in addition to the desired oneto-one adduct, 5-phenylhexene-2. The latter can be purified, if desired, by distillation before being utilized as feed for the present process.

Referring now to FIG. 1, the 5-phenylhexene-2 feed enters through line and, in one embodiment of the invention, is heated and vaporized in heater 11 and then enters reaction Zone A. This zone is provided with a solid acidic alkylation catalyst capable of causing cyclization or ring closure as shown in Equation I.

(1H cu @CH2-CH2-CH2=CH2-CH3 @(b *3 fi-phenylhexene-2 'lii-dihmthyltctralin The catalyst for this reaction can be any suitable solid acidic catalyst such as silica-alumina, silica-magnesia, silica-alumina-zirconia, acidic crystalline zeolites and the like. Solid phosphoric acid catalysts are particularly suitable for this reaction. This type of catalyst is commercially available and has been described, for example, in US. Pat. 2,585,899, issued Feb. 12, 1952, G Langlois,

4 and in US. Pat. 3,504,045, issued Mar. 31, 1970, E. J. Scharf et al. and patents cited therein.

The cyclization reaction in Zone A, depicted by Equation I, preferably is carried out in the presence of hydrogen which is admitted to the reactor through recycle line 12. Conditions for this reaction include temperatures in the range of 200450 C. and pressures in the range of 0-500 p.s.i.g. The presence of the hydrogen is beneficial in that it tends to suppress the formation of high boiling by-products (C dimer alkylate) and improve the selectivity of the reaction for obtaining 1,4dimethyltetralin.

Reaction products leave Zone A as a vapor through line 13 and are sent, without being condensed, to line 15 and into dehydrogenation Zone B. Reaction temperatures in this zone are in the range of 300-500 C. and generally higher than in Zone A. Accordingly a heater 14 connected with line 13 is provided to raise the temperature of the mixture of hydrogen and 1,4-dimethyltetralin (typically 240 C. from Zone A) to the desired level for dehydrogenation (typically 430 C. average). Since this reaction is endothermic, means (not shown in FIG. 1) may be provided for supplying heat to the reactants in Zone B to prevent the temperature from dropping too low before the dehydrogenation reaction is completed.

A solid dehydrogenation catalyst is employed in Zone B to catalyze the reaction given by Equation H.

@i 2H CH3 CH3 1,4-dimethyltetralin L l-DMN As shown, the reaction produces two moles of hydrogen for each mole of reactant. This reaction thus serves as the source of supply of hydrogen used in Zone A, as well as that used in Zone C in the embodiment illustrated by FIG. 1. This reaction is carried out at a temperature level in the range of 300500 C. and a pressure in the range of 0-500 p.s.i.g. Any solid dehydrogenation catalyst capable of effecting the dehydrogenation and exhibiting a reasonable life under these conditions can be used, including catalysts composed of noble metals on carriers such as reforming catalysts, chromia-alumina and the like. However, it is preferable to use a platinum-on-alumina dehydrogenation catalyst and particularly platinum on non-acidic alumina, as this kind of catalyst will exhibit a particularly long life under the conditions employed in dehydrogenation Zone B.

The reaction in Zone B not only brings about the desired dehydrogenation but results in an unexpected benefit, in that any C dimeric alkylation product which may have been formed in Zone A tends to be reconverted under the conditions of Zone B to C material including dimethylnaphthalene.

In the embodiment of FIG. 1 the reaction mixture from dehydrogenation Zone B passes as a vapor through line 16 to line 18 and into isomerization Zone C where temperatures in the range of 275500 C. and pressures of 0-500 p.s.i.g. are maintained. Since it is usually desirable to carry out the isomerization reaction at a temperature (typically 330 C.) lower than the average temperature of Zone B, means are provided, illustrated as heat exchanger 17, for regulating the temperature of the mixture of 1,4-DMN and hydrogen passing through line 16.

The catalyst employed in Zone C is a solid acidic isomerization catalyst and can be of the same types as described with respect to Zone A. Suitable acidic catalysts include silica-alumina or silica-alumina-zirconia as taught in US. Pat. 3,336,411 listed above and crystalline zeolite catalysts as described in aforesaid U.S. applications Ser. No. 207,870 and Ser. No. 208,001. Solid phosphoric acid catalysts as employed in the processes of aforesaid U.S. Pats. 2,585,899 and 3,504,045 likewise are suitable.

The reaction in Zone C causes the 1,4-DMN to isomerize to only two of its isomers, since a shift of a methyl substituent cannot occur between a 2-position and a 3-position or from one ring to the other on the naphthalene nucleus, as disclosed in aforesaid U.S. Pat. 3,109,036. The only isomerization possibilities that could be involved in this reaction are illustrated by Equation III.

In this reaction the 1,4-isrner is highly unfavored so that almost all of it will convert to the other two isomers. At equilibrium at a temperature level of 330 C, the product will contain only 1% or less of the 1,4-isomer and the proportion of 2,3-DMN to 1,3-DMN will be of the order of 25:74. At lower temperatures a somewhat higher proportion of 2,3-DMN to 1,3-DMN would be obtained (e.g. 35:64- at 70 C., as shown in the Suld et al. reference cited above). A small amount of disproportionation and other reactions also may occur during this isomerization, resulting in the formation of, for example, a total of 5-10% of monomethyland trimethylnaphthalenes, 1-3% of other DMNs, and say 13% of C dimers of DMNs. If desired, these various byproducts can be separated from the DMNs by subjecting the product from Zone C to fractional distillation (not shown) before the mixed DMNs are further processed.

In the simplified illustration of the process in FIG. 1, the isomerization product from Zone C passes through line 19 and condenser 20 to gas-liquid separator 21. Condenser 20 is operated so as to liquefy the isomer products without reducing temperature enough to cause any crystallization. The liquid product is removed from separator 21 through line 22 and sent to an isomer separation step illustrated schematically at 23. This separation can be done in any suitable manner, such as by crystallization, adsorption or selective complexation, so as to selectively recover 2,3-DMN from the isomer mixture. Selective crystallization is the preferred method of recovery, inasmuch as the 2,3-DMN has a melting point of 105 C. while 1,3-DMN and 1,4-DMN have melting points of -4 C. and 8 C., respectively. The 2,3-DMN is removed via line 24 as the desired product. The other isomers preferably are recycled, as indicated by dashed line 25, for further conversion to 2,3-DMN in Zone C.

The gas phase, composed principally of hydrogen, is removed from the top of separator 21 and partly vented from the system through valve 26 and line 27. The rest of the hydrogen flows through line 28 to compressor 29 where it is recompressed for reuse in the system. The pressurized hydrogen is then recycled through line 30 back to a step preceding Zone C. Preferably the hydrogen is sent through valve 31 and line 12 back to Zone A for reuse in each of Zones A, B and C. However, the process can also be operated by closing valve 31, opening valve 32 and recycling the hydrogen from line 30 through line 15 to Zone B. Also part of the hydrogen from line 30 optionally can be recycled through valve 33 and line 18 into Zone C. This will permit adjustment of the hydrogen to hydrocarbon ratio in Zone C independently of such ratio in the efiiluent from Zone B and allow maintenance of a selected ratio regardless of variations in the amount of DMN recycled through dashed line 25.

By operating the process as described for FIG. 1, no condensers and separators are needed between Zones A, B and C, and only the single compressor 29 is required for supplying hydrogen to each of the three zones at the pressures at which they are operated. The arrangement thus minimizes plant investment and operating costs.

The process of FIG. 1 can be varied by carrying out the cyclization in Zone A by a liquid phase reaction, in which case the feed from line is heated to the selected cyclization temperature in heater 11 but not vaporized.

Suitable conditions for such liquid phase reaction employing a solid phosphoric acid catalyst include a temperature of 200-275 C., LHSV of 0.1-5 and preferably 0.3-2, and whatever pressure is needed to maintain the hydrocarbons as a liquid. Another suitable type of catalyst for liquid phase cyclization comprises acidic ion exchange resins, in which case reaction temperatures in the range of 70-140 C., preferably -125 0, should be employed. The liquid phase cyclization reaction can also be carried out by means of other types of catalysts such as acidic crystalline zeolites, siliceous cracking catalysts or liquid catalysts such as hydrofluoric or sulfuric acid. When a liquid catalyst is used, the temperature for effecting the cyclizing reaction can be relatively low, e.g. 0-lO0 C. The liquid reaction product in any of these cases is then vaporized in heater 14 and dehydrogenated in Zone B as previously described. In this embodiment valve 31 is closed and the hydrogen is recycled through valve 32 and line 15 to Zone B.

Regardless of whether the cyclization reaction in Zone A is carried out in vapor or liquid phase, the resulting 1,4- dimethyltetralin generally is associated with an appreciable amount of various types of byproducts. The following tabulation shows typical compositions for the cyclization In spite of the various byproducts formed, such material can be processed under the conditions described for Zone B without rapid deactivation of the dehydrogenation catalyst when a platinum-on-alumina catalyst is employed. Particularly long catalyst lives can be achieved when the catalyst used is platinum-on-alumina which is nonacidic or has been rendered non-acidic by treatment with a solution of an alkaline salt such as lithium carbonate. Under the conditions maintained in Zone B the C dimer material is converted almost entirely to C products inclifding DMN, as previously mentioned. Furthermore the olefinic components (phenylhexenes) rapidly become saturated in Zone B and thus also do not tend to cause carbonaceous deposits that deactivate the catalyst.

In the embodiment illustrated in FIG. 2 reaction Zones A and B are the same as the corresponding zones in FIG. 1 and they are operated with reactants in vapor phase under the same conditions utilizing the same types of catalysts. The process of FIG. 2 difiers in that the product from Zone B is condensed and the hydrogen is recycled back to Zone A. The feed enters through line 40 and is vaporized in heater 41 and then introduced into Zone A where reaction occurs according to Equation I. The mixtures of 1,4-dimethyltetralin and hydrogen passes through line 43, heater 44 and line 45 into the top of Zone -B wherein reaction occurs according to Equation H. The reaction mixture flows from Zone B through line 46 and condenser 47 and into gas-liquid separator 48.

The 1,4-DMN from separator 48 passes via line 50 through temperature regulating means 51 for adjusting the temperature to Whatever level is desired for reaction in Zone C. If desired, this reaction can be carried out utilizing HF, or HF with a small proportion of BF as catalyst, as described in aforesaid U.S. Pat. 3,109,036 and the J. Org. Chem. article by Suld et al. This procedure involves dissolving the 1,4-DMN in an inert solvent, such as n-hexane or benzene, and contacting the solution at 0-100" C. with HF or HF and a small amount of BF to effect the isomerization reaction. The use of such lower temperature has the advantage of increasing the 2,3-isdmer content of the isomerization product. Alternatively, the isomerization in Zone C can be carried out as a vapor phase reaction in the manner described in connection with FIG. 1.

The isomerization product from Zone C flows through line 53 to an isomer separation zone, indicated at 54, from which 2,3-DMN is removed as a product. The other isomeric material recovered, which is mainly 1,3-DMN, preferably is recycled through dashed line 55 to Zone C for further conversion.

The gas phase flows from the top of separator 48 into compressor 49 and then is recycled through lines 56 and 42 to cyclization Zone A. Excess hydrogen can be removed from the system via line 57 and valve 58'. When the isomerization reaction of Zone C is carried out in vapor phase as described in conjunction with FIG. 1, a portion or all of this excess hydrogen from line 57 can advantageously be used in Zone C, being admitted thereto as indicated by dashed line 52. In such cases means (not shown in FIG. 2) should be provided for separating the hydrogen from the isomer products obtained through line 53.

A preferred manner of practicing the invention is illustrated by FIG. 3. In this embodiment a single reactor column is used for reaction Zones B and C, the column being provided with dehydrogenation catalyst in the form of three superposed beds 68, 69 and 70 in its upper portion (Zone .B) and a bed 71 of acidic isomerization catalyst in its lower portion (Zone C). Heating means, illustrated as coils 72 and 73, are provided between the Zone B beds to supply heat to compensate for the endothermic nature of the dehydrogenation reaction. The catalysts for Zones A, B and C are the same as described for the respective zones in FIG. 1.

The feed enters the FIG. 3 system via line 60, is vaporized in heater 61 and is introduced, together with recycled hydrogen from line 63, through line 62 to cyclization Zone A which operates under the same conditions as previously described. Preferred conditions for this reaction include a temperature of 210-250 C. and pressure of 20-200 p.s.i.g. Other conditions generally used include a liquid hourly space velocity (LHSV) of 0.1-5, preferably 0.3-2, and a hydrogen to hydrocarbon mole ratio of 0.1-10, preferably 1-5.

While the main reaction in Zone A is cyclization as depicted by Equation I, interaction between the C feed molecules and the C cyclization product can result in the formation of C dimeric alkylation products. These high boiling products, if present in substantial amounts during the next step of the process, tend to cause progressive deactivation of the catalyst and help to shorten its life. The presence of the recycled hydrogen in Zone A is beneficial in that it minimizes the amount of these higher boiling products formed and avoids any necessity of removing them prior to the next reaction step.

The mixture from Zone A passes through line 64, heater 65, line 66 and into the top of column 67. Heat is supplied by heater 65 and coils 72 and 73 so as to maintain a temperature throughout Zone B in the range of 300-500 C. and preferably of 375-450 C. Preferably a pressure of 20-200 p.s.i.g. is maintained in this zone. Other conditions include LHSV of 0.1-10, preferably 3-7, and hydrogen to hydrocarbon mole ratio of 0.1-10, preferably 1-5. While various kinds of dehydrogenation catalysts can be used for eifecting the dehydrogenation in Zone B, it is distinctly preferable to employ a platinum-on-alurnina catalyst and especially non-acidic platinum-on-alumina, as this catalyst will provide an unexpectedly long life under these reaction conditions. One way of preparing such catalyst is described in aforesaid United States Patents 3,255,268 and 3,325,551.

Under the conditions maintained in Zone B not only does dehydrogenation take place as shown in Equation II but also an unexpected benefit occurs, namely, the reconversion of C dimeric alkylation material to C products. This material converts partly back to dimethylnaphthalene and in part to a by-product tenatively identified as phenylhexane. This fortuitous result is beneficial in that it eliminates the presence of high boiling materials in the reaction mixture effiuent from Zone B and improves the yield of desired product. The conditions in Zone B also rapidly effect hydrogenation of any olefinic bonds, converting any uncyclized phenylhexene to phenylhexane, thus avoiding any possibility of this type of component undergoing polymerization and contributing to catalyst deactivation.

The vapor mixture from the bottom of Zone B passes directly to the top of isomerization catalyst bed 71 constituting Zone C. The temperature of the mixture is regulate-d to the level desired for the Zone C reaction by the introduction of recycle composed mainly of 1,3-DMN through line 85 and sparger 84. For this purpose a heat exchanger 74 is provided in the recycle stream so that the stream temperature can be adjusted as required to secure the proper temperature in bed 71. The broad temperature range for the isomerization reaction is 275-500 C., with the preferred temperature being 325-375 C. Pressure in Zone C is determined by the pressure maintained in Zone B, and the hydrogen to hydrocarbon ratio depends upon the proportion of these components in the vapor effiuent flowing from the bottom of bed 70. LHSV for Zone C falls in the range of 0.1-5 and preferably 0.5-2.

The effluent from the bottom of column 67 passes through line 75 and condenser 76 to gas-liquid separator 77. Condenser 76 is operated so as to reduce the temperature sufiiciently to liquefy the dimethylnaphthalenes without causing the 2,3-DMN to crystallize. The hydrogen phase from the top of the separator is partly vented from the system through valved line 78, and the rest is recompressed in compressor 79 and then recycled through line 80, heater 81 and lines 63 and 62 to Zone A.

While the efiiuent from column 67 is composed mainly of 2,3-DMN and 1,3-DMN with a small amount of 1,4- DMN, it also generally will contain small amounts of monomethyland trimethyl-naphthalenes, other DMN isomers and C dimeric material due to the occurrence of disproportionation and other side reactions in the preceding steps. Those by-products which boil substantially below and above the DMNs can be separated, if desired, by providing means (not shown in FIG. 3) for distilling the efliuent obtained via line 75 before the 2,3-DMN is recovered therefrom in the next step.

The hydrocarbon isomers from the bottom of separator 77 are set to a crystallization step for selectively crystallizing the 2,3-isomer, which has the highest melting point (105 C.), from the 1,3-DMN (M.P.=-4 C.). These isomers will form a eutectic mixture, composed of about 10% 2,3-DMN and 90% 1,3-DMN, which crystallizes at 8 C. The temperature for this crystallization step therefore should be above 8 C. and typically will be in the range of 5 C. to 20 C. The separated 2,3-DMN is removed as indicated by line 83 as the desired product, while the mother liquor comprising largely the 1,3-isomer together with minor proportions of 2,3-DMN and 1,4- DMN is recycled through heat exchanger 74 and line 85 to isomerization Step (C).

The invention thus provides an efficacious way of effecting the multistep conversion of 5-phenylhexene-2 to 2,3-dimethylnaphthalene employing a minimum of processing equipment. Once the feed has been vaporized, the reactants can flow as vapors successively through each of the conversion Zones A, B and C without any intermediate condensation, so that handling of liquids is not required until after isomerization Step (C). Hydrogen generated in Zone B can beneficially be used in Zones A and C with only one compressor being required for the entire system. The hydrogen in Zone A tends to minimize the formation of heavy material (C dimeric alkylation products), and furthermore any small amount thereof that is formed advantageously becomes reconverted to lower boiling material in Zone B.

The use of platinum-on-alumina as the dehydrogenation catalyst for the Zone B reaction provides unexpected advantages in that unusually good catalyst lives can be obtained. While commercial platinum reforming catalysts can be used for this dehydrogenation reaction, it is distinctly preferable to employ platinum on non-acidic alumina. For example, in the reaction of Zone A effluent having the typical composition hereinbefore listed, in the presence of platinum on non-acidic eta or gamma alumina and hydrogen at a pressure of 150 p.s.i.g., temperature of 430 C., H to hydrocarbon mole ratio of 10:1 and liquid hourly space velocity of 4-5, a catalyst life in excess of 6000 lbs. feed/lb. of catalyst can be obtained. The selectivity in conversion of 1,4-dirnethyltetralin to 1,4-DMN typically will be above 95%. For the present purpose catalyst life is considered to be coextensive with the period during which the 1,4-dimethyltetralin is converted to 1,4-DMN to the extent of at least 90% of the equilibrium value at the temperature of operation.

By way of comparison, when the Zone B reaction is carried out under essentially the same conditions except that other catalysts such as palladium on carbon, palladium on alpha or gamma alumina or chromia on alumina are substituted for the non-acidic Pt-on-A1 O catalyst, catalyst life values of only to 105 lbs./lb. of catalyst are typical.

The invention claimed is:

1. Process for converting S-phenylhexene-Z to 2,3-dimethylnaphthalene which comprises:

(A) passing a feed composed mainly of -phenylhexene2 in vapor form over a solid acidic catalyst at a temperature in the range of 200-450 C. and a pressure of 0-500 p.s.i.g. to efiect cyclization and form 1,4-dimethyltetralin;

(-B) contacting the reaction product from Step (A) in vapor form with a solid dehydrogenation catalyst at a temperature in the range of 300-500 C., in the presnce of hydrogen and at a pressure in the range of 0-500 p.s.i.g. to convert 1,4-dimethyltetralin to 1,4-dimethylnaphthalene by dehydrogenation;

(C) contacting the resulting vapor mixture of 1,4-dimethylnaphthalene and hydrogen with a solid acidic isomerization catalyst at a temperature in the range of 275-500 C. and a pressure in the range of 0-500 p.s.i.g. to form a mixture of dimethylnaphthalenes including 2,3-dimethylnaphthalene;

(D) separating hydrogen from the reaction product containing 2,3-dimethylnaphthalene;

(E) and recycling separated hydrogen to a step in the process preceding Step (C).

2. Process according to claim 1 wherein said separated hydrogen is recycled to Step (A).

3. Process according to claim 1 wherein said separated hydrogen is recycled to Step (B).

4. Procss according to claim 1 wherein 2,3-dimethylnaphthalene is recovered from the reaction product from Step (C) by selective removal from isomeric dimethylnaphthalene.

5. Process according to claim 4 wherein said isomeric dimethylnaphthalene is recycled to Step (C) for isomerization to the 2,3-isomer.

6. Process according to claim 5 wherein Steps (B) and (C) are carried out in a single reactor column containing a bed of said dehydrogenation catalyst adjacent its inlet end and a bed of said acidic isomerization catalyst adjacent its other end, and wherein said isomeric dimethylnaphthalene is recycled by being introduced into the vapor stream between the respective beds.

7. Process according to claim 6 wherein said dehydrogenation catalyst comprises platinum on non-acidic alumina.

8. Process according to claim 1 wherein Step (A) is carried out at a temperature of 210-250 C. and a pres- 10 sure of 20-200 p.s.i.g., the vapor reaction product there from is heated and reacted in Step (B) at a temperature of 375-450 C. and a pressure of 20-200 p.s.i.g., and the vapor reaction product from Step (B) is reacted in Step (C) at a temperature of 325-375 C. and a pressure of 20-200 p.s.i.g.

9. Process according to claim 1 wherein 2,3-dimethylnaphthalene is recovered from the reaction product from Step (C) by selective crystallization thereof from isomeric dimethylnaphthalene, and said isomeric dimethylnaphthalene is admixed with vapor reaction product from Step (B) to form a mixture having a temperature of 325-375 C. for isomerization in Step (C).

10. Process according to claim 1 wherein the catalyst in Step (B) comprises platinum on non-acidic alumina.

11. Process for converting S-phenylhexene-Z to 2,3- dimethylnaphthalene which comprises:

(A) passing a feed composed mainly of S-phenylhexene-2 in vapor form and in admixture with recycled hydrogen hereinafter specified over a solid acidic catalyst at a temperature in the range of ZOO-450 C. and a pressure of 0-500 p.s.i.g. to effect cyclization and form 1,4-dimethyltetralin;

(B) contacting the resulting vapor mixture of l,4-dimethyltetralin and hydrogen with a solid dehydrogenation catalyst at a temperature in the range of 300-500 C. and a pressure in the range of 0-500 p.s.i.g. to convert 1,4-dimethyltetralin to l,4-dimethylnaphthalene by dehydrogenation;

(C) separating hydrogen from the reaction product containing 1,4-dimethylnaphthalene;

(D) passing separated hydrogen to Step (A) as said recycled hydrogen;

(E) and contacting said product containing 1,4-dimethylnaphthalene with an acidic isomerization catalyst under isomerizing conditions to form a mixture of dimethylnaphthalene including 2,3-dimethylnaphthalene.

12. Process according to claim 11 wherein Step (A) is carried out at a temperature of 210250 C. and a pressure of 20200 p.s.i.g., and the vapor reaction product therefrom is heated and reacted in Step (B) at a temperature of 375450 C. and a pressure of 20-200 p.s.i.g.

13. Process for converting 5-phenylhexene-2 to 2,3- dimethylnaphthalene which comprises:

(A) contacting a feed composed mainly of S-phenylhexene-2 with an acidic catalyst under cyclizing conditions to effect cyclization and form 1,4-dimethyltetralin;

(B) contacting the reaction product from Step (A) in vapor form with a solid dehydrogenation catalyst at a temperature in the range of 300-500" C., in the presence of hydrogen and at a pressure in the range of 0-5-00 p.s.i.g. to convert 1,4-dimethyltetralin to 1,4-dimethylnaphthalene by dehydrogenation;

(C) contacting the resulting vapor mixture of 1,4-dimethylnaphthalene and hydrogen with a solid acidic isomerization catalyst at a temperature in the range of 275500 C. and a pressure in the range of 0-500 p.s.i.g. to form a mixture of dimethylnaphthalenes including 2,3-dimethylnaphthalene;

(D) separating hydrogen from the reaction product containing 2,3-dimethylnaphthalene;

(E) and recycling hydrogen to Step (B).

14. Process according to claim 13 wherein 2,3-dimethylnaphthalene is recovered from the reaction product from Step (C) by selective removal from isomeric dimethylnaphthalene.

15. Process according to claim 14 wherein said isomeric dimethylnaphthalene is recycled to Step (C) for isomerization to the 2,3-isomer.

16. Process according to claim 15 wherein Steps (B) and (C) are carried out in a single reactor column containing a bed of said dehydrogenation catalyst adjacent its inlet end and a bed of said acidic isomerization catalyst adjacent its other end, and wherein said isomeric dimethe ylnaphthalene is recycled by being introduced into the vapor stream between the respective beds.

17. Process according to claim 16 wherein said dehydrogenation catalyst comprises platinum on non-acidic alumina.

18. In a process for converting S-phenylhexene-Z to dimethylnaphthalene, the steps which comprise:

(A) converting the -phenylhexene-2 mainly to 1,4-

dimethyltetralin by contacting same with an acidic catalyst under cyclizing conditions at which a cyclization reaction occurs to give reaction product composed mainly of 1,4-dimethyltetralin but also containing C dimeric alkylation product;

(B) contacting said reaction product in vapor phase and in the presence of hydrogen with a solid dehydrogenation catalyst comprising platinum-on-alumina at a temperature of 300-500 C., whereby 1,4-dimethyltetralin is dehydrogenated to 1,4-dimethylnaphthalene and said C dimeric alkylation product is converted to C products including dimethylnaphthalene;

(C) recovering hydrogen produced by the dehydrogenation reaction and recycling same in the process to Step (A) or Step (B).

19. A process according to claim 18 wherein said dehydrogenation catalyst is platinum on non-acidic alumina.

20. A process according to claim 18 wherein the catalyst in Step (A) is a solid phosphoric acid catalyst.

21. A process according to claim 18 wherein the catalyst in Step (A) is a solid phosphoric acid catalyst and the temperature is 210-250 C., and wherein the catalyst in Step (B) is platinum on non-acidic alumina and the temperature is 375450 C.

22. In a process in which 5-phenylhexene-2 is cyclized in the presence of an acidic catalyst to 1,4-dimethyltetralin and in which process by-product higher than C is formed, the improvement which comprises subjecting said by-product to dehydrogenation conditions in the presence of a solid dehydrogenation catalyst to convert at least some of said by-product to dimethylnaphthalene.

23. Process according to claim 22 in which said byproduct is a C 24. Process according to claim 22 in which said dehydrogenation catalyst is platinum on non-acidic alumina.

References Cited UNITED STATES PATENTS 3,244,758 4/1966 Eberhardt 260-668 B 3,109,036 10/1963 Suld et a1 260668 A 3,336,411 8/1967 Benham 260668 F 3,207,801 9/1965 Frilette et al 260-668 F 3,428,698 2/1969 Peterson 260-668 F CURTIS R. DAVIS, Primary Examiner US. Cl. X.R. 260-668 A 

